Soil carbon sequestration by three legume pastures is greater in deep soil than in surface soil

Biogeosciences,13,527–534,2016/13/527/2016/doi:10.5194/bg-13-527-2016©Author(s) Attribution3.0License.Soil carbon sequestration by three perennial legume pastures is greater in deeper soil layers than in the surface soilX.-K.Guan1,N.C.Turner2,3,L.Song4,Y.-J.Gu2,T.-C.Wang1,and F.-M.Li2,31Collaborative Innovation Center of Henan Grain Crops,Agronomy College of Henan Agricultural University, Zhengzhou,Henan450002,China2State Key Laboratory of Grassland Agro-Ecosystem,Institute of Arid Agroecology,School of Life Sciences, Lanzhou University,Lanzhou730000,China3The UW A Institute of Agriculture and Centre for Plant Genetics and Breeding,The University of Western Australia,M080,Perth,W A6009,Australia4Medical College of Northwest University for Nationalities,Lanzhou730030,ChinaCorrespondence to:T.-C.Wang(wtcwrn@)and F.-M.Li(fmli@)Received:22April2015–Published in Biogeosciences Discuss.:8July2015Revised:18December2015–Accepted:4January2016–Published:28January2016Abstract.Soil organic carbon(SOC)plays a vital role as both a sink for and source of atmospheric carbon.Revegeta-tion of degraded arable land in China is expected to increase soil carbon sequestration,but the role of perennial legumes on soil carbon stocks in semiarid areas has not been quan-tified.In this study,we assessed the effect of alfalfa(Med-icago sativa L.)and two locally adapted forage legumes, bush clover(Lespedeza davurica S.)and milk vetch(Astra-galus adsurgens Pall.)on the SOC concentration and SOC stock accumulated annually over a2m soil profile.The re-sults showed that the concentration of SOC in the bare soil decreased slightly over the7years,while7years of legume growth substantially increased the concentration of SOC over the0–2.0m soil depth.Over the7-year growth period the SOC stocks increased by24.1,19.9and14.6Mg C ha−1un-der the alfalfa,bush clover and milk vetch stands,respec-tively,and decreased by4.2Mg C ha−1in the bare soil.The sequestration of SOC in the1–2m depth of the soil accounted for79,68and74%of the SOC sequestered in the2m deep soil profile under alfalfa,bush clover and milk vetch,respec-tively.Conversion of arable land to perennial legume pasture resulted in a significant increase in SOC,particularly at soil depths below1m.1IntroductionConcerns about global warming and increasing atmospheric greenhouse gas concentrations(CO2,CH4,and N2O)have led to questions on the role of soils as a source or sink for car-bon.Excluding carbonated rocks,soils constitute the largest surface carbon pool,approximately1500Gt,equivalent to almost twice that in the terrestrial biomass and three times that in the atmosphere(IPCC,2000).Globally,soil cultiva-tion has resulted in the loss of more than40Pg C,at a rate of about1.6Pg C yr−1,to the atmosphere during the1990s (Smith,2008).Chinese agricultural soils have also lost30–50%or more of the soil carbon pool(Lal,2004a).Soil organic carbon(SOC)is a significant component of the global carbon stocks(Chen et al.,2008).Globally,24% of the SOC stock has been lost through the conversion of for-est to cropland(He et al.,2011)and59%through the con-version of pasture to cropland(Zhu,1994).Fortunately,the loss of SOC can be slowed down by implementing crop man-agement practices such as conservation tillage(Lal,2004b; Puget and Lal,2005),converting degraded arable land to perennial grassland(Gentile et al.,2005),using diverse ro-tations,and introducing legume and grass mixtures into the rotation(Lal,2002,2004b,c).In the USA,the revegetation of highly-erodible crop-land or other environmentally sensitive areas to resource-conserving vegetation for a period of10to15years increasedPublished by Copernicus Publications on behalf of the European Geosciences Union.the SOC content in the upper3m of soil at an average rate of1.1Mg C ha−1yr−1(Osborn,1993).This conservation re-serve program(CRP)also significantly increased the soil C pool(Staben et al.,1997)and provided multiple bene-fits both environmentally and economically(Munson et al., 2012;Wu and Lin,2010).Like the CRP program in the USA, a program of soil and water conservation,namely“Grain for Green”was implemented on the Loess Plateau of China in1999to alleviate land degradation.The program of eco-environmental revegetation focused on the recovery of dam-aged ecosystems(Wang et al.,2010)by the use of peren-nial vegetation to control soil erosion,increase the stocks of SOC and prevent the occurrence of dry layers in the loess soils(Fu et al.,2010).Alfalfa(Medicago sativa L.)has been widely grown on the Loess Plateau to increase livestock pro-duction and improve water-use efficiency and soil fertility through high forage production,and for its ability to decrease soil erosion andfix atmospheric N(Guan et al.,2013).Addi-tionally,locally adapted legume species such as bush clover (Lespedeza davurica S.)and milk vetch(Astragalus adsur-gens Pall.)have been widely grown as cover crops or wind-breaks to protect the soil from water and wind erosion in arid and semiarid regions of northern China(Wang,2003;Xu et al.,2006).The“Grain for Green”program has reduced wind and water erosion of marginal arable land and is expected to significantly contribute to soil C sequestration.Recent stud-ies on the Loess Plateau have investigated and estimated the changes in SOC stocks in the top1m of soil as a result of revegetation of regional watersheds(Fu et al.,2010;Wang et al.,2011;Yan et al.,2007;Zhang et al.,2013).How-ever,deep-rooted perennial legumes may penetrate deeper in the soil profile than1m,likely underestimating the SOC-sequestration potential of these forage legumes in northwest China(Smith et al.,2015).The objective of this research was to assess the effect of alfalfa and two locally adapted forage legumes,bush clover and milk vetch,on the SOC concentration and SOC stock ac-cumulated annually for7years over a2m soil profile.The SOC content in the2m soil profile was measured at the end of each growing season to quantify the SOC concen-tration and stock under the locally adapted forage legumes and alfalfa,and to provide specific information for estimat-ing the SOC sequestration potential of an important agricul-tural area.The hypothesis tested was that long-term growth of deep-rooted perennial legumes will increase soil organic C,provide a feed source for animals and provide a sink for atmospheric carbon.2Materials and methods2.1Experimental site descriptionThe experiment was conducted at the Changwu Agro-ecological Experiment Station on the LoessPlateau Figure1.Mean monthly temperature and precipitation from2004–2010and the long-term mean at the experimental site at Changwu Agricultural Research Station,Shaanxi Province,China.(35◦12 N,107◦40 E),Shaanxi province,China,from2004 to2010.The level site is located at1220m above sea level. The climate is semiarid,with an annual mean temperature of 9.1◦C and a mean annual precipitation of579mm(1979–2003)with rainfall concentrated in the period from July to September.Precipitation and temperature data were recorded at the Changwu Meteorological Station,20m from the ex-perimental site.The groundwater table is50–80m below the soil surface,making it unavailable for plant growth.Prior to the establishment of this experiment,the site was planted to winter wheat for many(at least20)years.For winter wheat production,the site was ploughed to a depth of0.3m twice a year,after harvest in early July and again in September be-fore sowing;only wheat stubble was returned to the soil,but 108kg N ha−1and276kg P2O5ha−1of fertilizer was applied each year before sowing.In2003,after the winter wheat was harvested,the site remained fallow for280days to allow moisture accumulation over the winter before the legumes were sown in May2004.Soil at the experimental site belongs to the Loess series. The texture in the top5m is a uniform silty clay loam(haplic greyxems,FAO-UNESCO,1988),with a mean sand,silt,and clay content of3.5,65.6,and30.9%,respectively.The soil physical characteristics do not significantly change in the up-per5m.The measured average bulk density of the soil in the upper2m is1.31g cm−3,does not change with depth,and the top0.3m contained1.55%total organic matter,0.106% nitrogen,and0.095%available phosphate prior to the com-mencement of the experiment in2004.Biogeosciences,13,527–534,/13/527/2016/2.2Treatments and measurement of abovegroundbiomassTwelve experiment plots,each4m by3m,were established in early May2004with one of three forage legume species, milk vetch(Astragalus adsurgens Pall.),alfalfa(Medicago sativa L.)and bush clover(Lespedeza davurica S.),and an unplanted control.Each legume species was grown as an evenly spaced monoculture(but assumed a more patchy dis-tribution with time)at a seeding density of25plants m−2, weeds were removed from all plots by hand.The plots were adjacent to each other.During the experimental period from 2004to2010,there was no irrigation or other form of sup-plementary water and the plots were not fertilized.The plants were not inoculated,but relied on the naturally occurring root nodule bacteria from previous growth of the three species on the experimental ck of nodulation was not ob-served to be a problem.Treatments were completely random-ized in three replicate blocks.Each year from2005–2010,measurements of above-ground biomass production for each legume were taken at the end May,July and September(in2004only one cut was made in September)by cutting the plants at ground level with hand-held shears in a randomly-selected1m×1m quadrat within each plot,but avoiding border areas.At the same time, the rest of the plot was also cut at the same height and the forage removed.The oven-dry weight was determined after drying at105◦C for0.5h and then further dried at75◦C for 48h(Guan et al.,2013).2.3Soil sampling and analysisSoil samples were taken with a cylindrical steel corer(di-ameter40mm and height200mm)at two random positions in each plot which were combined into one composite sam-ple per plot before analysis.Each plot was sampled from the surface to2m deep at depths of0–0.3,0.3–0.6,0.6–1.0,1.0–1.5and1.5–2.0m before sowing on10May2004and at the end of each growing season(29October)from2004to2010. The soil samples were air-dried,roots and organic debris re-moved,ground and sieved through a2mm sieve,then stored at room temperature before analyzing the SOC.The concentration of SOC(in g kg−1)was measured us-ing the wet dichromate oxidation procedure(Moinuddin and Khanna-Chopra,2004).Briefly,a0.5g soil sample was di-gested with5mL of1N K2Cr2O7and5mL of concentrated H2SO4at150◦C for0.5h,followed by titration of the digest with standardized FeSO4.2.4SOC stock calculation and statistical analysesSoil organic C stock was calculated as Eq.(1):C stock=SOC×ρ×H×10,(1)Table1.Annual aboveground biomass production of the three legume species,milk vetch,alfalfa and bush clover,from2004to 2010.Adopted from Guan et al.(2013)and used with permission.Aboveground biomass production Year(t ha−1)Milk vetch Alfalfa Bush clover 2004 2.2Ac 2.3Ad0.2Bd200514.1Ba20.2Aa 5.3Cc200614.3Ba22.2Aa7.8Ca2007 6.8Bb9.3Ac 6.4Bbc2008 5.6Bb13.4Ab7.3Bab20097.2Bb12.4Ab7.8Ba2010 5.8Bb10.8Abc7.4Bab2004–2010Mean8.0B13.0A 6.0C2004–2010Total56.0B90.7A42.1C Data in each column with a different lower-case letter are significantly different (P<0.05)and data in each row with a different capital letter are significantlydifferent(P<0.05).where SOC is the SOC concentration(g kg−1)in each soil layer,ρis the bulk density(g cm−3)of the soil,and H is the depth of each layer.The data were analyzed by analysis of variance(ANOV A), and means were compared using the LSD at P<0.05to characterize the differences among treatments.PROC GLM (General Linear Model)in the Statistical Analysis System (SAS Institute,Cary,NC,version8.02)software was used to assess the temporal changes in SOC stock and the rate and amount of SOC sequestered.3Results3.1Meteorological conditionsThe average monthly mean temperature from June to Au-gust,the primary growth period for the legumes,was about 20◦C.Monthly mean temperatures were about1◦C warmer than the long-term mean throughout the experimental pe-riod(Fig.1).Over the experimental period,the total an-nual precipitation varied from470mm in2006to583mm in2010,and was below the long-term mean in all years ex-cept2010when the rainfall was similar to the long-term mean(Fig.1).Rainfall from July to September accounted for 55–60%of total annual precipitation,while rainfall in the legume-growing season(from April to October)was about 90%(range from84%in2009to96%in2005)of the total annual precipitation(Fig.1).3.2Aboveground forage biomass productionThe results of the aboveground biomass production over the 7years have been reported by Guan et al.(2013).Briefly,the annual production of milk vetch increased from2.2t ha−1in/13/527/2016/Biogeosciences,13,527–534,2016Table2.Results of the ANOV A for soil organic carbon concentra-tion as affected by legume species,soil depth and experimental year. The bare soil plot is considered as a legume species in the analysis. The SAS PROC ANOV A was used in the analysis.Factors d f F value Pr>FSpecies338.520.0003Depth41649.40<0.0001Year731.68<0.0001Species·Depth12 5.65<0.0001Species·Year21 5.96<0.0001Depth·Year28 3.20<0.0001Species·Depth·Year840.950.6053thefirst year to14.3t ha−1in2006and then decreased,al-falfa increased from2.3t ha−1in thefirst year to a maxi-mum of22.2t ha−1in2006and then decreased,while bush clover increased from0.2t ha−1in thefirst year to7.8t ha−1 in2009and did not decrease significantly thereafter(Ta-ble1).Total aboveground biomass production over the ex-perimental period was highest in alfalfa at91t ha−1(equiv-alent to43Mg C ha−1assuming a C to dry weight ratio of 0.475,Magnussen and Reed,2004)compared to56t ha−1 (27Mg C ha−1)in milk vetch and42t ha−1(20Mg C ha−1) in bush clover(Table1).3.3SOC concentration over the soil profileThe legumes significantly(P<0.001)increased the SOC concentration at each soil depth,and this effect varied with legume species and experimental year(Table2).The initial concentration of SOC in May2004decreased with increas-ing soil depth(Fig.2).In the upper0–0.3m of soil,the initial SOC concentration was8.0±0.03g kg−1,while it was only 3.3±0.27g kg−1in the1.5–2.0m soil layer(Fig.2).Com-parison of the SOC concentration between the initial values on10May2004and those at the end of the experimental pe-riod in October2010showed that the concentration of SOC in the bare soil decreased slightly over the7years,while 7years of legume growth substantially increased the concen-tration of SOC over whole2m soil depth.There were large increases in the concentration of SOC at0.6–1.0,1.0–1.5and 1.5–2.0m soil depth and a small,but significant,increase in the upper0.3m of the soil in bush clover,but not in milk vetch and alfalfa.No significant changes were observed after 7years at the0.3–0.6m depth(Fig.2).3.4SOC stock over the experimental periodSOC stock was calculated by converting SOC concentra-tion to the amount of SOC per soil layer per unit area. The SOC stock in2004varied from21.4±0.85Mg C ha−1 in the0.3–0.6m soil layer to32±0.14Mg C ha−1and 32±0.68Mg C ha−1at0–0.3and1.0–1.5m depthrespec-Figure2.Concentration of soil organic carbon(SOC)with depth in May2004(IV)and in October2010under three forage legumes: milk vetch,alfalfa and bush clover,and bare soil(CK).Bars give+ one standard error of the mean(n=3).tively(Fig.3).In the bare soil,the SOC stock decreased at all depths across the experimental period,but only de-creased significantly at−0.36Mg C ha−1yr−1(P<0.05)in the1.5–2.0m layer(Fig.3d),presumably from the decay and turnover of the wheat roots accumulated over the many years of wheat production prior to the planting of the legumes.In the legume plots,the SOC stock increased linearly with time (2004–2010)in the0–0.3,0.6–1.0,1.0–1.5m and1.5–2.0m soil layers,but not in the0.3–0.6m soil layer(Fig.3).The change in SOC stock over the7years was greatest at soil depths below1.0m in all three species and was greatest in the alfalfa plots with rates of1.35Mg C ha−1yr−1at a depth of 1.0–1.5m(P<0.001;Fig.3d),and1.39Mg C ha−1yr−1at a depth of1.5–2.0m(P<0.001;Fig.3e).The highest accumu-lation of SOC stock occurred at a depth of1.0–1.5m in bush clover where it averaged1.58Mg C ha−1yr−1(P<0.001; Fig.3d).Over the full0–2.0m depth,the SOC stock at the be-ginning of the experiment in2004was137Mg C ha−1,de-creased to133Mg C ha−1in the bare soil plots,while it in-creased to151,157and161Mg C ha−1under the milk vetch, bush clover and alfalfa stands,respectively,by the end of the experiment in2010(Fig.4).The SOC stock increased more under the stand of alfalfa than milk vetch,but there was no significant difference between alfalfa and bush cloverBiogeosciences,13,527–534,/13/527/2016/Figure3.Change with stand age in soil organic carbon amount(stock)per hectare at soil depths of0–0.3(a),0.3–0.6(b),0.6–1.0(c), 1.0–1.5(d)and1.5–2.0m(e)under milk vetch,alfalfa,bush clover and bare soil(CK).Note the soil layers vary in depth.Data are means ±one standard error of the mean(n=3)when larger than the symbol.Linear regressionsfitted when significant andfitted regressions given./13/527/2016/Biogeosciences,13,527–534,2016Figure4.The soil organic carbon amount(stock)under milk vetch, alfalfa,bush clover and under bare soil(CK)over the upper2m of the soil profile.The lower case letters indicate significant differ-ences(P<0.05)between forage types and bare soil(CK)within a year.IV denotes initial value,the soil organic carbon stock in May 2004.Bars give+one standard error of the mean(n=3).(Fig.4).When calculated over the full2m soil layer,over the 7-year growth period,the SOC stocks increased by24.1,19.9 and14.6Mg C ha−1under the alfalfa,bush clover and milk vetch stands,respectively,and decreased by4.2Mg C ha−1 under bare soil(Fig.5).In the1.0–2.0m soil layer the stocks of SOC increased by19.1,13.6and10.8Mg C ha−1,under the alfalfa,bush clover and milk vetch stands,respectively, that is,by79,68and74%of the increases over the whole soil profile(Fig.5).4DiscussionWhile the unplanted plots lost SOC over the7years of the study,particularly over thefirst5years,all three for-age legumes more than countered this loss,so that over the7years the SOC increased on average by2.1,2.8and 3.4Mg C ha−1yr−1,to sequester15,20and24Mg C ha−1 under the milk vetch,bush clover and alfalfa stands,respec-tively(Fig.5).The legumes increased the SOC primarily at depth(Fig.2)with between68and79%of the SOC being sequestered below1m.An increase in SOC stock is usually associated with the production,turnover,sloughing off of epidermal cells,and exudation of soluble carbon compounds by the roots,partic-ularly the turnover and exudation byfine roots(Luo et al., 1995).A high rate of turnover offine roots and a high rate of exudation of carbon by the roots influences the stability of plant C in soil and the accumulation of SOC(Shahzad et al., 2015).Although the root biomass was not measured in this study,root biomass is usually associated with aboveground biomass.If the root:shoot biomass ratio is known,theroot Figure5.Change in soil organic carbon amount(stock)in different soil layers under milk vetch,alfalfa,bush clover and bare soil(CK) from May2004to October2010.Different letters indicate signifi-cant differences(P<0.05)between total carbon stocks. biomass and root carbon accumulated in the soil can be es-timated from the aboveground biomass and compared with the changes in SOC over the same ing an average root:shoot ratio of0.77:1in the three species(Chen and Nie,1978;Cheng et al.,2004;Fan et al.,2015;Sun et al., 2001),the91t ha−1of aboveground biomass(43Mg C ha−1) produced over the7-year life of the alfalfa pasture would result in the production of70t ha−1of root biomass or 33Mg C ha−1of root carbon,significantly higher than the measured increase in SOC stock in the upper2m of the soil of24Mg C ha−1.Similarly,using an aboveground biomass of56t ha−1,the estimated production of root biomass by milk vetch would be43t ha−1and the estimated root car-bon would be20Mg C ha−1,also higher than the increase of15Mg C ha−1in SOC stock measured under milk vetch. However,using the same root:shoot ratio of0.77:1,the aboveground biomass of bush clover of42t ha−1would pro-duce an estimated root biomass of32t ha−1and a produc-tion of root carbon of15Mg C ha−1,compared with the mea-sured SOC of20Mg C ha−1.Thus the actual accumulation of SOC was about70%of that estimated from the above-ground biomass in alfalfa and milk vetch,but was33% higher than that estimated in bush clover.The difference be-tween the estimated root carbon accumulated and the mea-sured SOC stock under alfalfa and milk vetch over the7-year period was presumably the result of losses by respiration by the roots and associated soil microbial populations.The observation that the measured values of SOC were higher than the root carbon production estimated from aboveground biomass may indicate that the root:shoot ratio was greater than0.77:1in bush clover(a root:shoot ratio of1:1would make the estimated and measured values the same),or the bush clover had a greater proportion and turnover offine roots than milk vetch and alfalfa,resulting in a greater ac-Biogeosciences,13,527–534,/13/527/2016/cumulation of SOC(Shahzad et al.,2015).This is consistent with Sun et al.(2001)who reported that thefine roots(root diameter<0.5mm)of bush clover accounted for42%of to-tal root biomass in0–0.3m soil layer,while thefine roots of milk vetch were only25%of total root biomass(Chen and Nie,1978).The accumulation of SOC by bush clover was particularly high in the upper0.3m of the soil(Figs.2and3).This accu-mulation of SOC in the upper soil layer may be attributable to the high accumulation of legume residues and litter(Zhou et al.,2006),or due to the proliferation and turnover of roots in this surface layer.Nevertheless,the sequestration of SOC in the upper0.3m of the soil in this study was sig-nificantly lower than Zhang et al.(2009)who reported that the SOC stocks in the upper0.3m of the soil increased by 16Mg C ha−1in10years from the conversion of a wet reed meadow to an irrigated alfalfa pasture in the Hexi Corridor of northwest China.This suggests that well-managed legume pastures in areas with higher precipitation and with appropri-ate fertilizer use could sequester significantly more SOC than in the present unirrigated and unfertilized legumes growing in a semiarid environment.An unexpected result from this study was the greater in-crease in SOC at soil depths from1–2m than above1m,ac-counting for79,68and74%of SOC sequestered through the whole2m of soil under alfalfa,bush clover and milk vetch, respectively.This suggests the presence of roots deep in the soil profile.Indeed,alfalfa and milk vetch have taproots that can penetrate to6.8and7.6m,respectively,in loess soils in6years(Cheng et al.,2004,2005),and the water extrac-tion patterns measured on the plots used in this study clearly showed the presence of roots throughout the upper2m of the soil profile(Guan et al.,2013).In bush clover in which the proportion of SOC below1m was smaller than in the other two species,the taproot predominates in the0–0.3m soil layer with coarse roots(root diameter>2mm)account-ing for48%of the total root biomass,andfine roots predom-inating below0.3m(Cheng et al.,2004,2005;Sun et al., 2001).Although alfalfa extracted more water below1m than the other two species,the water extraction patterns do not suggest a greater root presence below1m in milk vetch than bush clover(Guan et al.,2013).We suggest that the greater increase in SOC at depth may be associated with a greater proliferation and turnover offine roots at depth,or alterna-tively may reflect the movement down the profile of soluble C compounds from the roots as a result of the movement of water after heavy rainfall events.The conversion of arable land that had been growing crops for many years to perennial legume pasture resulted in a sig-nificant increase in SOC,particularly at soil depths below 1m.All three legume species increased the SOC in the top 2m of the soil profile,but the increase was greatest in alfalfa and least in milk vetch.While the production of aboveground biomass was least in bush clover,the SOC sequestration in the soil profile was not significantly different from alfalfa,indicating that carbon sequestration in the soil is not simply associated with aboveground biomass production in a sys-tem in which the forage is removed for animal feed,as in the present study.Further study of root biomass production, turnover offine roots and exudation of carboxylic acids and other carbon compounds by the roots of the legume species would be a valuable step in understanding the differences in carbon sequestration by the three species. Acknowledgements.This research was supported by grants(no. 31470496and no.31471452)from the National Natural Science Foundation of China,the National Key Technology R&D Program of China(Projects2012BAD04B07and2012BAD14B08)and the Innovation Team Program of the Ministry of Education of China(no.IRT_13R26).This research was also supported by the Fundamental Research Funds for the Central Universities (lzujbky-2015-ct02),and the Program of Introducing Talents of Discipline to Universities(“111Project2007B051”).We are grateful to Xiao-Hong Zhang for her help in the laboratory.NCT thanks the Centre for Plant Genetics and Breeding and the UWA Institute of Agriculture at the University of Western Australia for financial support.This work is a part of the doctoral dissertation of thefirst author,Xiao-Kang Guan,at Lanzhou University.Edited by:S.FontaineReferencesChen,B.and Nie,C.:Research of Astragalus adsurgens Pall.root system,Journal of Gansu Agricultural University,2,71–75,1978 (in Chinese with English abstract).Chen,H.,Shao,M.,and Li,Y.:Soil desiccation in the Loess Plateau of China,Geoderma,143,91–100,2008.Cheng,J.,Wan,H.E.,and Wang,J.:Alfalfa growth and its relation with soil water status in loess hilly and gully region,J.Appl.Ecol.,16,435–438,2005(in Chinese with English abstract). Cheng,J.M.,Wan,H.E.,Wang,J.,and Yong,S.P.:Over depletion and recovery of soil moisture on Astragalus adsurgens grasslands in the loess hilly-gully region,Acta Ecologica Sinica,24,2979–2983,2004(in Chinese with English abstract).Fan,J.W.,Du,Y.L.,Turner,N.C.,Wang,B.R.,Fang,Y.,Xi,Y., Guo,X.R.,and Li,F.M.:Changes in root morphology and phys-iology to limited phosphorus and moisture in a locally-selected cultivar and an introduced cultivar of Medicago sativa L.grow-ing in alkaline soil,Plant Soil,392,215–226,2015.Fu,X.,Shao,M.,Wei,X.,and Horton,R.:Soil organic carbon and total nitrogen as affected by vegetation types in Northern Loess Plateau of China,Geoderma,155,31–35,2010.Gentile,R.M.,Martino,D.L.,and Entz,M.H.:Influence of peren-nial forages on subsoil organic carbon in a long-term rotation study in Uruguay,Agriculture,Ecosys.Environ.,105,419–423, 2005.Guan,X.K.,Zhang,X.H.,Turner,N.C.,Xu,B.C.,and Li,F.M.:Two perennial legumes(Astragalus adsurgens Pall.and Les-pedeza davurica S.)adapted to semiarid environments are not as productive as lucerne(Medicago sativa L.),but use less water, Grass Forage Sci.,68,469–478,2013./13/527/2016/Biogeosciences,13,527–534,2016He,Z.,Huang,C.,Zheng,H.,Zhou,J.,Pang,J.,Li,X.,and Wang, L.:Holocene loess and its deposition dynamics in the upper reaches of the Huaihe River,J.Geogr.Sci.,21,561–573,2011. IPCC:Land-use,land-use change,and forestry,in:Land use,land-use change,and forestry:a special report of the intergovernmen-tal panel on climate change,edited by:Watson,R.T.,Noble,I.R.,Bolin,B.R.,Ravindranath,N.H.,Verardo,D.J.,and Dokken,D.J.,Cambridge University Press,UK,2000.Lal,R.:Soil carbon dynamics in cropland and rangeland,Environ.Pollut.,116,353–362,2002.Lal,R.:Offsetting China’s CO2Emissions by Soil Carbon Seques-tration,Climatic Change,65,263–275,2004a.Lal,R.:Soil carbon sequestration impacts on global climate change and food security,Science,304,1623–1627,2004b.Lal,R.:Soil carbon sequestration to mitigate climate change,Geo-derma,123,1–22,2004c.Luo,Y.,Meyerhoff,P.A.,and Loomis,R.S.:Seasonal patterns and vertical distributions offine roots of alfalfa(Medicago sativa L.), Field Crops Res.,40,119–127,1995.Mangnussen,S.and Reed, D.:Modeling for Estimation and Monitoring,available at:/fileadmin/user_ upload/national_forest_assessment/images/PDFs/English/KR2_ EN_10.pdf,2004.Moinuddin,K.-C.and Khanna-Chopra,R.:Osmotic Adjustment in Chickpea in Relation to Seed Yield and Yield Parameters,Crop Sci.,44,449–455,2004.Munson,S.,Lauenroth,W.,and Burke,I.:Soil carbon and nitro-gen recovery on semiarid Conservation Reserve Program lands, J.Arid Environ.,79,25–31,2012.Osborn,T.:The conservation reserve program:Status,future and policy options,J.Soil Water Conserv.,48,271–279,1993. Puget,P.and Lal,R.:Soil organic carbon and nitrogen in a Mollisol in central Ohio as affected by tillage and land use,Soil Till.Res., 80,201–213,2005.Shahzad,T.,Chenu,C.,Genet,P.,Barot,S.,Perveen,N.,Mougin,C.,and Fontaine,S.:Contribution of exudates,arbuscular mycor-rhizal fungi and litter depositions to the rhizosphere priming ef-fect induced by grassland species,Soil Biol.Biochem.,80,146–155,2015.Smith,P.:Land use change and soil organic carbon dynamics,Nutr.Cycl.Agroecosys.,81,169–178,2008.Smith,P.,House,J.I.,Bustamante,M.,Sobocká,J.,Harper,R., Pan,G.,West,P.,Clark,J.,Adhya,T.,Rumpel,C.,Paustian, K.,Kuikman,P.,Cotrufo,M.F.,Elliott,J.A.,McDowell,R., Griffiths,R.I.,Asakawa,S.,Bondeau,A.,Jain,A.K.,Meers-mans,J.,and Pugh,T.A.M.:Global Change Pressures on Soils from Land Use and Management,Glob.Change Biol.,1–21, doi:10.1111/gcb.13068,2015.Staben,M.,Bezdicek,D.,Fauci,M.,and Smith,J.:Assessment of soil quality in conservation reserve program and wheat-fallow soils,Soil Sci.Soc.Am.J.,61,124–130,1997.Sun,Q.,Han,J.,Gui,R.,Li,Z.,and Liu,G.:Biomass in Lespedeza dahurica S.,Grassland of China,4,21–26,2001(in Chinese with English abstract).Wang,G.:Differences in leafδ13C among four dominant species in a secondary succession sere on the Loess Plateau of China, Photosynthetica,41,525–531,2003.Wang,Y.,Shao,M.A.,and Shao,H.:A preliminary investigation of the dynamic characteristics of dried soil layers on the Loess Plateau of China,J.Hydrol.,381,9–17,2010.Wang,Y.,Fu,B.,Lü,Y.,and Chen,L.:Effects of vegetation restora-tion on soil organic carbon sequestration at multiple scales in semi-arid Loess Plateau,China,Catena,85,58–66,2011. Wu,J.and Lin,H.:The effect of the conservation reserve program on land values,Land Economics,86,1–21,2010.Xu,B.,Gichuki,P.,Shan,L.,and Li,F.:Aboveground biomass production and soil water dynamics of four leguminous forages in semiarid region,northwest China,South African Journal of Botany,72,507–516,2006.Yan,H.,Cao,M.,Liu,J.,and Tao,B.:Potential and sustainabil-ity for carbon sequestration with improved soil management in agricultural soils of China,Agriculture,Ecosys.Environ.,121, 325–335,2007.Zhang,C.,Liu,G.,Xue,S.,and Sun,C.:Soil organic carbon and total nitrogen storage as affected by land use in a small watershed of the Loess Plateau,China,Eur.J.Soil Biol.,54,16–24,2013. Zhang,T.,Wang,Y.,Wang,X.,Wang,Q.,and Han,J.:Organic carbon and nitrogen stocks in reed meadow soils converted to alfalfafields,Soil Till.Res.,105,143–148,2009.Zhou,Z.,Sun,O.J.,Huang,J.,Li,L.,Liu,P.,and Han,X.:Soil carbon and nitrogen stores and storage potential as affected by land-use in an agro-pastoral ecotone of northern China,Biogeo-chemistry,82,127–138,2006.Zhu,X.:The formation and significance of loess-soil struction pro-file,J.Soil Water Conserv.,8,1–9,1994(in Chinese with English abstract).Biogeosciences,13,527–534,/13/527/2016/。